When should we use the semi-colon and why? Is it not just an overgrown comma?
The semi-colon is an alluring mysterious creature, rare and lovely. It should be treated with tenderness and consideration.
There are at least two uses of the semi-colon that I can think of. The most obvious is as a means of dividing up a list. Why not use a comma for this purpose? Well, it's when you have a list that is made up not of single word items but of longer phrases that you might divide them up with the semi-colon. Sometimes this can allow you to use commas as well, within those phrases, without confusion.
This is a bit like having a quick one with a geisha, mind.
The other time you would use a semi-colon is in the middle of a sentence, to divide two phrases. Those occasions when a comma is not quite enough but a full stop is too much, when there is a hint of causality in the air, that's when you might dream of inserting a semi-colon. If you hesitated over whether you might say 'because' or 'then' but decided that would be too clumsy; the delicate application of a semi-colon will probably fit your bill. If you are writing something that is separate from the phrase which came before it but has in it a longing to be attached; the semi-colon will act like the flirtatious meeting of eyes across a crowded room to connect the two. It offers a little pause for reflection; before plunging on into action again.
(Actually in that last sentence I ought to have used a comma but I couldn't resist!)